Stacked magnetic arrangement

ABSTRACT

A magnetic arrangement includes a stack of members including at least two permanent magnet members and at least one intermediate member of soft magnentic material having a permeability higher than that of said permanent magnet members and sandwiched between said permanent magent members.

United States Patent [1 1 [111 3,889,220 Spodig 1 June 10, 1975 [54] STACKED MAGNETIC ARRANGEMENT 3,206,655 9/1965 Reijnst 335/306 X [76] n entor: einrich spodig, Netteberge 84, 3,535,200 10/1970 Bergstom 335/302 X 4711 Bork, Germany [22] Filed: June 26, 1973 Primary ExaminerG. Harris 1 pp NO: 372,707 Attorney, Agent, or FzrmM1chae1 S. Str1ker [30] Foreign Application Priority Data July 3, 1972 Germany 2232613 [57] ABSTRACT [52] US. Cl 335/306; 335/302 A magnetic arrangement includes a stack of members [51 Int. Cl. HOlf 7/02 including at least two permanent magnet members and [58] Field of Search 335/302, 303, 304, 306, at least one intermediate member of soft magnentic 335/284 material having a permeability higher than that of said permanent magnet members and sandwiched between [5 6] References Cited said permanent magent members.

UNITED STATES PATENTS 2,947,921 8/1960 Watelet 335/306 X 26 Claims, 10 Drawing Figures PATENTEDJUHIO I975 3,889,220 SHEET 1 FIGJ STACKED MAGNETIC ARRANGEMENT BACKGROUND OF THE INVENTION In order to understand the method according to the invention, it may be advisable to review certain basic aspects of magnetic theory, which will be referred to in discussing the dimensioning of permanent magnets for the purpose of achieving high flux densities and high field strengths at the magnet poles.

Tha ratio L /D L is the length of the permanent magnet (in centimeters). D is the diameter (in centimeters) of a rodshaped permanent magnet of circular cross section. When the permanent magnet has a rectangular, oval or other non-circular cross section. the D is the diameter of a circle having an area equal to the cross-sectional area of the permanent magnet of non-circular cross section.

The produt L 'H L has been defined above. HM (expressed in oersteds) is the magnetic field intensity within the permanent magnet, as indicated by the working point on the second quadrant portion of the hysteresis curve of the magnet. The product L 'H is equal to the magmetomotive force ofthe magnet, sometimes referred to as the magnetic potential drop across the magnet poles.

The product L,;H,

If the permanent magnet forms part of a magnetic circuit including an air gap, L is the length of the air gap (in centimeters) measured along the direction of flux travel, and H, (in oersteds) is the intensity of the magnetic field in the air gap. The product L 'H is equal to the so-called magnetic potential drop across the air gap. It is useful to remember that if the air gap is of very short length, the intensity of the air-gap magnetic field l-I will have approximately the same numerical value as the flux density B of the magnetic field in the air gap, provided that the field intensity H, is expressed in oersteds and the flux density is expressed in gausses.

If the permanent magnet having a magnetomotive force L 'H is formed into a magnetic circuit including a very short air gap, the magnetic potential drop across the air gap L 'H will be equal to the magnetomotive force of the magnetizing permanent magnet. Expressed mathematically: L H L H, Moreover, if the gaussian system of units is used, H, is approximately equal to B, so that the relationship becomes: L ,-H, L, B

The product F 'B This product is equal to the total flux (1) flowing through the permanent magnet, and in the gaussian system of units is expressed in maxwells. B is the flux density of such total flux. also referred to as the induction at the magnet poles, and F is the cross-sectional area (in cm of the path travelled by the magnetic flux d). The flux density or induction can be conversely defined as B (in gausses) d) (in maxwells) divided by F," CHILE).

As the field of producing magnetic materials gradually began its development over 100 years ago. the practice initially was to makeuse almost exclusively of magnetic carbon steels, By the time of World War I, magnetic alloyed carbon steels had been developed,

notably carbon steels alloyed with Cr, Co, Mo, Ni and W0. The next stage in this evolution included the development and immediate wide use of Al-Ni and Al-Ni-Co steels, which were produced in isotropic form by precipitation hardening and also in anisotropic form, directionally magnetized. In the last twenty years, a major new development has occurred, reducing greatly the importance of-magnetic steel in the field of magnetic materials. This latest development is the use of non-metallic magnetic materials, such as barium, strontium and lead ferrites and other such materials. These can be produced in both isotropic and anisotropic form and furthermore can be directionally magnetized. The success of these new materials in the marketplace has been very great.

The continuing development in the field of ferromagnetic steels and ferrimagnetic materials was mainly characterized by the continual increase in the coercive forces of the magnetic materials produced. The coercive force H rose gradually from 30 to 60 to to 200 to 300 to 500 to 600 to 1,000 to 1,500 to 2,000 and up to 4,000 oersteds. The high coercive force of these newly developed materials, which is a desirable property, is accompanied by an undesirable decrease in the residual induction B,.. The materials developed exhibited higher and higher magnetomotive forces L -H and had higher and higher specific magnetization resistances H along the length L of permanent magnets formed from the newly developed materials. The fiux density or induction value at the poles became smaller and smaller.

All this occurs because, as the remanance decreases and the coercive force increases, which is the trend in the development of the new magnetic materials in question, the hysteresis curves of the materials become shorter in vertical (induction) direction and wider in horizontal (field intensity) direction. The increasing resistance to magnetization of the new materials, and especially the increased magnetomotive force exhibited by them, leads to the production of permanent magnets of much shorter length than was the case with permanent magnets formed from the materials popular in earlier days. Ceramic and oxidic barium, strontium and lead ferrites (H 1,000-2,000 oersteds) are produced predominantly as flat disks and plates, and the new metallic cobalt, iron and platinum alloys (H 4,000

oersteds) are even formed as foils, with the magnetic flux-path length in the case of foils, disks and plates being extremely short relative to the transverse dimensions of the permanent magnet.

A disadvantage of the newly developed magnetic materials is that they are more expensive than the magnetic materials they are supplanting, such as the older aluminum, nickel, and cobalt steel alloys, etc.

SUMMARY OF THE INVENTION It is the general object of the present invention to provide a new way of using the newly developed magnetic materials of high coercive force in a manner which very greatly reduces the quantity of such magnetic material required to bring about a desired magnetomotive force and/or flux density for a permanent magnet of a given size and shape.

This extremely advantageous result can be brought about, according to the present invention, by providing a magnet arrangement comprising, in combinations, a stack of members including at least two permanent magnet members and at least one intermediate member of soft magnetic material having a permeability higher than that of the permanent magnet members and sandwiched between the permanent magnet members. Advantageously, the magnet arrangement has the' form of a stack of permanent magnet members of high coercive force, made from relatively expensive magnetic material, alternating with members of higher permeability, such as iron, and having a much lower cost.

As will be explained below, it has been found that much of the high-cost high-coercive-force material of a magnet ofa given size and shape can be replaced with low-cost high-permeability material such as iron or other low-cost soft magnetic material, without any substantial alteration in the pole flux density of the magnet and in the magnetic field intensity of the magnet.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows the second quadrant of the hysteresis curves for several different magnetic materials;

FIGS. 2a and 2b are views of a plate of magnetic material, in elevation and in side view;

FIGS. 3-8 illustrate six different stack-like magnet arrangements; and

FIG. 9 is a graph of induction and magnetic field intensity versus magnet length for each of the different permanent magnet arrangements shown in FIGS. 3-8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As an example of the manner in which the inventive concept can be realized, permanent magnet arrangements were formed using barium ferrite magnets, having the magnetization curve labelled Ba-Fe300K in the set of graphs shown in FIG. 1. The set of graphs in FIG. 1 constitute the second-quadrant hysteresis curves of a number of different magnetic materials. The barium ferrite magnets employed in the illustrative embodiment were anisotropic and directionally magnetized. Because of the shape of the hysteresis curve for this material, and in particular because a very high-intensity magnetic field is required to magnetize the material, the optimal L /D ratio for this material is less than or equal to 0.5.

As mentioned before, it is the general purpose of the invention to form permanent magnets of a given size and shape using only a relatively small amount of expensive high-coercive-force magnetic material without any substantial loss in magnetic field intensity or induction; conversely expressed, it is the general purpose of the invention to form permanent magnets, using a given amount of expensive high-coercive-force magnetic material, having a higher field intensity and induction than prior-art permanent magnets using the same amount of identical high-coercive-force magnetic material.

To form such an improved permanent magnet arrangement, use is made, in the illustrated embodiments, of permanent-magnet plate-shaped members having a square cross section of 9.8 X 9.8 cm, with a magnetic length L of 2.5 cm and a central mounting bore having a diameter of 1.6 cm, one such magnet plate being illustrated in FIGS. 2a and 2b. Also, use was made of additional plates having the same cross section but made of soft magnetic material (for example Armco iron having quality characteristics St. 34, St. 37 or St 60). Moreover, these additional plates of soft magnetic material were formed in different thicknesses, namely 1 cm, 2 cm, 2.5 cm, 3 cm and 4 cm. The effective diameter D of these various plates. com puted on the basis of an imagined circle having an area equal to the cross-sectional area of the plates, was I 1 cm. The magnetic length L of the permanent magnet plates, such as the one shown in FIGS. 2a and 2b, was 2.5 cm. Accordingly, for the permanent magnetplates of ferrite material, the ratio L /D 0.44. This value lies within the optimal range mentioned above of LM/DM S 0.5.

To establish a standard against which to compare the properties of the magnet arrangements of the invention, magnet arrangements were formed by simply stacking together different numbers of ferrite magnet plates such as shown in FIG. 2. The field intensity and induction of one such plate was measured. Then the field intensity and induction of a stack of two such plates was measured, then three such plates, and so on, up to a stack of sixteen ferrite permanent magnet plates such as shown in FIG. 3. In each stack, all the magnet members had the same north-south orientations. The stack of sixteen plates shown in FIG. 3 had a length of 40 cm. With each thusly formed stack, the flux density or induction B (in gausses) was measured by measuring the voltage in a coil drawn out of the neutral zone of the respective formed stack; this kind of technique is very common and is known to persons familiar with the principles of voltage induction. The measurement of the magnetic field intensity H (measured in oersteds) was accomplished using a Hall probe at the pole surfaces of the respective magnet stacks.

Using the just-described stacks of ferrite permanent magnet members as a standard of comparison, five other stacks were gradually built up, formed from identical such ferrite permanent magnet members alternating with plates of soft easily magnetizable magnetic material (such as Armco iron having quality characteristics St. 34, St. 37 or St. 60). In the stack shown in FIG. 4, the intermediate plates of soft magnetic material had a thickness of 1 cm. In the stack shown in FIGS. 5-8, the intermediate plates of soft magnetic material had respective thicknesses of 2 cm, 2.5 cm, 3 cm and 4 cm.

Each of the six stacks shown in FIGS. 3 8 was built up, plate by plate, and after the addition of each plate to each stack, measurements were taken of the field intensities and flux densities of the different stacks. These results are shown in FIG. 9, which is a graph of flux density B (in gausses) and also of field intensity H (in oersteds) versus stack length L The curves are derived from discrete measurements but the points of the graphs have been connected by smooth lines to facilitate visualization. In each of FIGS. 3-8, above the respective magnet stack, there is depicted the key for correlating the different stacks with the different curves appearing in FIG. 9.

It is noted again that in all the stacks shown in FIGS. 3-8, all the plates, both the ferrite permanent magnet plates and also the plates of soft magnetizable material, have the same cross section. Also, in each of the six stacks depicted, the north-south orientations of all the permanent magnet members of the stack are the same. With each of the five stacks shown in FIGS. 4-8, the stacks were built up in length, plate by plate, until they had a length approximately equal to 40 cm. the maximum length to which the reference stack of FIG. 3 was built up.

Turning now to the graph of FIG. 9, it is noted that the upper set of curves represents the variation of flux density B with varying stack length for all six stacks depicted. The lower set of curves represents the variation of magnetic field intensity H with varying stack length for all six stacks depicted. In the lower set of curves representing field intensity H, it will be noted that for the stacks shown in FIGS. 4-8, there are two curves for each stack such as curves 4a and 4b, 5a and 5b, etc. The curves 4a-8u depict the field intensities of the respective stacks when the upper end plate is a ferrite permanent magnet plate; the curves 4b8b depict the field intensities of the respective stacks when the upper end plate is a plate of soft magnetic material. As mentioned. before, interpolations have been made to yield continuous curves, instead of a point graph. to facilitate visualization of the results achieved.

It is noted that in FIG. 9, the designations 5M, 6M, 7M, etc., on the upper set of curves indicate the number of permanent magnet members in the respective stack when the stack had been built up to the length indicated on the horizontal axis.

Study of the graph reveals the surprising result that when the six stacks are each built up to a length of about 40 cm, the flux density B exhibited by the stacks of FIGS. 4-8 was only slightly less than that of the stack of FIG. 3 composed entirely of expensive ferrite permanent magnet material. The slight reduction in flux density is of course not an advantage per se. However, it is to be noted that a substantial reduction in the amount of expensive ferrite permanent magnet material required has been accomplished by forming the stacks as in FIGS. 4-8. Whereas the solid stack of FIG. 3 employed sixteen ferrite magnets, the stacks of FIGS. 4-8 respectively emdployed only eleven, eight, seven, seven and six ferrite magnets. Expressed in percentages, the reduction in the amount of expensive ferrite permanent magnet material required to bring about substantially similar results amounted to about 31.3% in the stack of FIG. 4 using 1 cm thick iron plates, about 48.2% in the stack of FIG. 5 using 2 cm thick iron plates. about 53.1% in the stack of FIG. 6 using 2.5 cm thick iron plates. about 56.271 in the stack of FIG. 7 using 3 cm thick iron plates. and about 62.5% in the stack of FIG. 8, using 4 cm thick iron plates. Inasmuch as the flux density with each of the stacks in FIGS. 48 was slightly lower than that of the stack of FIG. 3, one or two additional permanent magnets with intermediate iron plates can be added to each of the stacks of FIGS. 48, if it is desired to incur no loss in flux density.

Results as surprising as those discussed above were observed with regard to the measured field intensity of the stacks according to the invention. These results are depicted in the two lower sets of curves 4a-8u and 4l 8h in FIG. 9. The field intensity H of the stacks using a sharply reduced amount of expensive ferrite permanent magnet material was only somewhat lower than that of the stack comprised of only ferrite permanent magnet members shown in FIG. 3.

The graphs 4a-8a and 4b8b do indicate, however. a significant difference in properties depending on whether or not the upper end plate (in the case of the measurements here taken) was'of ferrite permanent magnet material or of iron. When the upper end plate was of ferrite permanent magnet material (the curves marked 3a-8a) the field intensity was markedly higher than when the upper end plate was of soft iron (the curves marked 4b-8h). Moreover, the discrepancy in H values increases significantly with increases in the thicknesses of the intermediate soft-iron plates. In the stack of FIG. 4, for example, where the intermediate soft-iron plates are only 1 cm thick, the difference between curves 4a and 4b, representing the difference resulting from the kind of material selected for the end plate, is not very great. On the other hand, with the stack of FIG. 8, where the intermediate plates are 4 cm I thick, substantially thicker than the permanent magnet plates, there is a much greater discrepancy between the height of curves 8a and 8b, indicating the significant difference in properties depending upon whether the upper end plate (in the example being described) is a ferrite permanent magnet plate or an iron plate. These results lead to the recommendation that when magnet stacks acccording to the present invention are formed, they should either have at their pole ends permanent magnet plates or else pole-shoe plates of soft magnetic material that are as thin as practicable. However, it is interesting and important to note that the recommended use of thin plates of soft magnetic material only 'concerns the use of such plates as end plates. There is no comparable restriction as to the thickness of the intermediate plates of soft magnetic material; these plates may be quite thick, to save on the expen sive permanent magnet material of the other plates, without too great a reduction in magnetic field intensity H. The intermediate plates to a great extent serve only to increase the magnetizability of the stack-type arrangement according to the invention, compared to the much more difficult magnetization of the permanent magnet material employed. In the presently illustrated embodiments, it has been found that optimal results are achieved if the magnetic length of the intermediate pole-shoe plates of soft magnetic material such as iron is kept less than or equal to the length of the permanent magnet plates, which in the illustrated embodiment means less than or equal to 2.5 cm.

It is noted parenthetically that in the illustrated stacks all the ferrite permanent magnet members of a particular stack have the same thickness and all the intermediate soft magnetic material members of a particular stack have the same thickness. However, this is of course not an absolute necessity.

A plurality of stacks of members like those shown, of different lengths, and of different configurations, can be combined to form larger magnet arrangements, with of course the north-south orientations of the various combined stacks being kept the same. The optimal (but not mandatory) relationship L /D 0.5 should still be observed, however, with respect to the total magnetic length and effective magnetic diameter of the resulting composite structure. The magnetic stacks according to the invention, or composite structures formed from a plurality of such stacks, can be used in combination with iron flux-return members to build magnetic circuitry of the type used in many different conventional applications.

To illustrate how the invention improves the magnetizability and reduces the expense of magnet ar' rangements comprising expensive permanent magnet material, several examples have been disclosed in which anisotropic directionally magnetized Ba Fe300K (see H6. 1) is used as the expensive permanent magnet material in question. However, other such expensive permanent magnet materials can of course be used, with all the benefits described above still being achieved. The great improvement in magnetizability, and/or the reduction in the amount of expensive permanent magnet material required, becomes ever the greater, the greater the coercive force of the permanent magnet material involved, i.e., becomes greater, the more difficult it is to magnetize the permanent magnet material in question. With the use of permanent magnet materials of extremely high coercive force, the inclusion of intermediate layers of highly permeable soft magnetic material such as iron, has a very marked effect upon the magnetizability of the composite.

It has already been explained that the inventive concept when implemented results in a reduction in the amount of expensive permanent magnetic material, for instance ferrite magnetic material, needed to construct a magnet having a certain field intensity and flux density. The inventive concept if furthermore advantageous insofar as the actual process of initially magnetizing the materials in question is concerned. For example, if a stack of unmagnetized hard ferrite members alternating with soft iron members is placed in a strong magnetizing field, the magnetomotive force of the magnetizing field can be substantially less than when a similar stack composed entirely of hard ferrite material is to be magnetized. The normal or virgin magnetization curve of the composite according to the invention, if initially unmagnetized, will have a shape representing a composite of the low-permeability characteristics of the hard ferrites and the high-permeability characteristics of the soft magnetic material. The resulting composite characteristic results in easier magnetization, that is, the use of smaller magnetomotive forces to effeet the initial magnetization, compared to what is required when similarly configurated magnets of hard ferrite material are being initially magnetized. Accordingly, lower magnetizing currents can be used to effect the initial magnetization. As will be understood by those skilled in the art, this advantageous increase in the magnetizability of the magnet arrangement Will result, in effect, even if the hard ferrite members are magnetized individually prior to formation of the stack structure according to the invention.

it will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the type described above.

While the invention has been illustrated and described as embodied in stacks of plate-shaped permanent magnet members alternating with intermediate members of magnetic material having a higher permeability than the permanent magnet members, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that. from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is setforth in the appended claims. 1. A magnet arrangement comprising, in combination, a stack of parts including at least two permanent magnet parts having respective first end faces of the same magnetic polarity and respective second end faces of the same magnetic polarity, said first end faces having a magnetic polarity opposite to the magnetic polarity of said second end faces, and an intermediate part made of a magnetic material having a magnetic permeability higher than the permeability of said permanent magnet parts and sandwiched between said permanent magnet parts and having a first end face juxtaposed with the first end face of one of said permanent magnet parts and an opposite second end face juxtaposed with the second end face of the other of said permanent magnet parts.

2. A magnet arrangement as defined in claim 1, wherein said stack is an elongated stack comprisedof permanent magnet parts alternating with intermediate parts having a permeability higher than the permeability of the permanent magnet parts, and wherein one end of said stack is a north-pole end and the other end of said stack is a south-pole end, and wherein each of said permanent magnet parts has a north-pole end facing towards said north-pole end of said stack and facing away from said south-pole end of said stack, and wherein each of said permanent magnet parts has a south-pole end facing towards said south-pole end of said stack and facing away from said north-pole end of said stack.

3. A magnet arrangement as defined in claim 1, wherein each of said parts is a single member.

4. A magnet arrangement as defined in claim 2, wherein each of said parts is a single member.

5. A magnet arrangement as defined in claim 2, wherein the end parts of said stack are permanent magnet parts.

6. A magnet arrangement as defined in claim 2, wherein one of the end parts of said stack is a permanent magnet parts.

7. A magnet arrangement as defined in claim 2, wherein the end parts of said stack are parts of magnetic material having a permeability higher than the permeability of said permanent magnet parts.

8. A magnet arrangement as defined in claim 1, wherein said permanent magnet parts are non-metallic. 9. A magnet arrangement as defined in claim 2, wherein said permanent magnet parts are non-metallic.

10. A magnet arrangement as defined in claim 1, wherein said permanent magnet parts are non-metallic and wherein said at least one intermediate part is metallic:

.11. A magnet arrangement as defined in claim 2, wherein said permanent magnet parts are non-metallic and wherein said intermediate members are metallic 12. A magnet arrangement as defined in claim 1, wherein said intermediate part is made of iron.

13. A magnet arrangement as defined in claim 2, wherein said intermediate parts are made of iron.

14. A magnet arrangement as defined in claim 1. wherein said permanent magnet parts are non-metallic and wherein said intermediate part is metallic and includes iron.

15. A magnet arrangement as defined in claim 2. wherein said permanent magnet parts are non-metallic and wherein said intermediate parts are metallic and include iron.

16. A magnet arrangement as defined in claim 1. wherein said parts are all plate-shaped parts.

17. A magnet arrangement as defined in claim I, wherein said intermediate part is at most as thick in direction of flux travel as said permanent magnet parts.

18. A magnet arrangement as defined in claim 2, wherein said intermediate parts are each no thicker in direction of flux travel than said permanent magnet parts.

19. A magnet arrangement as defined in claim 1, wherein all said parts are of congruent cross-sectional outline.

20. A magnet arrangement as defined in claim 2, wherein all said parts are of congruent cross-sectional outline.

21. A magnet arrangement as defined in claim 1, wherein said intermediate part has a cross-section in direction transverse to flux travel different from the cross-sections of said permanent magnet parts.

22. A magnet arrangement as defined in claim 2, wherein at least one of said intermediate parts has a cross-section in direction transverse to flux orientation different from the cross-sections of said permanent magnet members.

23. A magnet arrangement as defined in claim 1, wherein said parts of said stack have aligned mounting bores.

24. A magnet arrangement as defined in claim 2, wherein said parts of said stack have aligned mounting bores.

25. A magnet arrangement as defined in claim 1; and further including an additional stack of parts including at least two permanent magnet members, and wherein said intermediate parts extends intermediate and is sandwiched between said two permanent magnet parts of said additional stack.

26. A magnet arrangement as defined in claim 2; and further including an additional stack of permanent magnet parts alternating with intermediate parts having a permeability higher than that of the permanent magnet parts, and wherein at least some of said intermediate parts are common to both stacks. 

1. A magnet arrangement comprising, in combination, a stack of parts including at least two permanent magnet parts having respective first end faces of the same magnetic polarity and respective second end faces of the same magnetic polarity, said first end faces having a magnetic polarity opposite to the magnetic polarity of said second end faces, and an intermediate part made of a magnetic material having a magnetic permeability higher than the permeability of said permanent magnet parts and sandwiched between said permanent magnet parts and having a first end face juxtaposed with the first end face of one of said permanent magnet parts and an opposite second end face juxtaposed with the second end face of the other of said permanent magnet parts.
 2. A magnet arrangement as defined in claim 1, wherein said stack is an elongated stack comprised of permanent magnet parts alternating with intermediate parts having a permeability higher than the permeability of the permanent magnet parts, and wherein one end of said stack is a north-pole end and the other end of said stack is a south-pole end, and wherein each of said permanent magnet parts has a north-pole end facing towards said north-pole end of said stack and facing away from said south-pole end of said stack, and wherein each of said permanent magnet parts has a south-pole end facing towards said south-pole end of said stack and facing away from said north-pole end of said stack.
 3. A magnet arrangement as defined in claim 1, wherein each of said parts is a single member.
 4. A magnet arrangement as defined in claim 2, wherein each of said parts is a single member.
 5. A magnet arrangement as defined in claim 2, wherein the end parts of said stack are permanent magnet parts.
 6. A magnet arrangement as defined in claim 2, wherein one of the end parts of said stack is a peRmanent magnet parts.
 7. A magnet arrangement as defined in claim 2, wherein the end parts of said stack are parts of magnetic material having a permeability higher than the permeability of said permanent magnet parts.
 8. A magnet arrangement as defined in claim 1, wherein said permanent magnet parts are non-metallic.
 9. A magnet arrangement as defined in claim 2, wherein said permanent magnet parts are non-metallic.
 10. A magnet arrangement as defined in claim 1, wherein said permanent magnet parts are non-metallic and wherein said at least one intermediate part is metallic.
 11. A magnet arrangement as defined in claim 2, wherein said permanent magnet parts are non-metallic and wherein said intermediate members are metallic.
 12. A magnet arrangement as defined in claim 1, wherein said intermediate part is made of iron.
 13. A magnet arrangement as defined in claim 2, wherein said intermediate parts are made of iron.
 14. A magnet arrangement as defined in claim 1, wherein said permanent magnet parts are non-metallic and wherein said intermediate part is metallic and includes iron.
 15. A magnet arrangement as defined in claim 2, wherein said permanent magnet parts are non-metallic and wherein said intermediate parts are metallic and include iron.
 16. A magnet arrangement as defined in claim 1, wherein said parts are all plate-shaped parts.
 17. A magnet arrangement as defined in claim 1, wherein said intermediate part is at most as thick in direction of flux travel as said permanent magnet parts.
 18. A magnet arrangement as defined in claim 2, wherein said intermediate parts are each no thicker in direction of flux travel than said permanent magnet parts.
 19. A magnet arrangement as defined in claim 1, wherein all said parts are of congruent cross-sectional outline.
 20. A magnet arrangement as defined in claim 2, wherein all said parts are of congruent cross-sectional outline.
 21. A magnet arrangement as defined in claim 1, wherein said intermediate part has a cross-section in direction transverse to flux travel different from the cross-sections of said permanent magnet parts.
 22. A magnet arrangement as defined in claim 2, wherein at least one of said intermediate parts has a cross-section in direction transverse to flux orientation different from the cross-sections of said permanent magnet members.
 23. A magnet arrangement as defined in claim 1, wherein said parts of said stack have aligned mounting bores.
 24. A magnet arrangement as defined in claim 2, wherein said parts of said stack have aligned mounting bores.
 25. A magnet arrangement as defined in claim 1; and further including an additional stack of parts including at least two permanent magnet members, and wherein said intermediate parts extends intermediate and is sandwiched between said two permanent magnet parts of said additional stack.
 26. A magnet arrangement as defined in claim 2; and further including an additional stack of permanent magnet parts alternating with intermediate parts having a permeability higher than that of the permanent magnet parts, and wherein at least some of said intermediate parts are common to both stacks. 